redstone_web_socket not being found?

[Meai]

pubspec.yml:

name: 'dart_server_test'
version: 0.0.1
description: A web server built using the shelf package.
#author: Your Name <[email protected]>
#homepage: https://www.example.com

environment:
  sdk: '>=1.0.0 <2.0.0'

dependencies:
  args: '>=0.10.0 <0.14.0'
  shelf: '>=0.6.0 <0.7.0'
  redstone: "^0.6.4"
  postgresql: "^0.3.3"
  redstone_web_socket: "0.1.0-beta.1"

pub get:

Resolving dependencies...
Package test has no versions that match >=0.12.1 <0.13.0 derived from:
- code_transformers 0.3.1 depends on version >=0.12.1 <0.13.0

[derrick56007]

I was able to get the package but adding the plugin and running the server resulted in an unhandled exception

Unhandled exception:
Concurrent modification during iteration: Instance(length:2) of '_GrowableList'.
#0      ListIterator.moveNext (dart:_internal/iterable.dart:338)
#1      Future.forEach.<anonymous closure> (dart:async/future.dart:384)
#2      Future.Future.sync (dart:async/future.dart:168)
#3      Future.doWhile.<anonymous closure> (dart:async/future.dart:411)
#4      _RootZone.runUnaryGuarded (dart:async/zone.dart:1338)
#5      _RootZone.bindUnaryCallback.<anonymous closure> (dart:async/zone.dart:1370)
#6      _RootZone.runUnary (dart:async/zone.dart:1404)
#7      _FutureListener.handleValue (dart:async/future_impl.dart:131)
#8      _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:637)
#9      _Future._propagateToListeners (dart:async/future_impl.dart:667)
#10     _Future._complete (dart:async/future_impl.dart:467)
#11     _SyncCompleter.complete (dart:async/future_impl.dart:52)
#12     Processor.parse.<parse_async_body>.<anonymous closure>.<<anonymous closure>_async_body> (package:redstone/src/processor.dart:54:9)
#13     _RootZone.runUnary (dart:async/zone.dart:1404)
#14     _FutureListener.handleValue (dart:async/future_impl.dart:131)
#15     _Future._propagateToListeners.handleValueCallback (dart:async/future_impl.dart:637)
#16     _Future._propagateToListeners (dart:async/future_impl.dart:667)
#17     _Future._addListener.<anonymous closure> (dart:async/future_impl.dart:342)
#18     _microtaskLoop (dart:async/schedule_microtask.dart:41)
#19     _startMicrotaskLoop (dart:async/schedule_microtask.dart:50)
#20     _runPendingImmediateCallback (dart:isolate-patch/isolate_patch.dart:96)
#21     _RawReceivePortImpl._handleMessage (dart:isolate-patch/isolate_patch.dart:149)